US President Donald Trump has claimed he has left standing orders for the US military to unleash massive retaliation against Iran if Tehran follows through on what he described as long-standing assassination threats against him.However, US law provides no mechanism for an automatic, preauthorised "dead man's switch" that would trigger military action upon the president's death.Instead, if Trump were assassinated, presidential authority would immediately pass to Vice President JD Vance under the 25th Amendment and the Presidential Succession Act of 1947. As commander in chief, Vance would decide whether to carry out Trump's desired response, modify it, or reject it altogether."The US has, for a whole variety of reasons, never utilised a technical dead man's switch," Garrett M Graff, author